KR101917478B1 - Hot formed parts and method for manufacturing the same - Google Patents

Abstract

A hot-rolled product obtained by hot press forming, wherein the hot-rolled product contains 0.25% or less of C (excluding 0%), 0.1 to 0.5% of Si, 0.9 to 1.8% of Mn, (Excluding 0%), Ti: not more than 0.05% (excluding 0%), P: not more than 0.05%, S: not more than 0.004%, N: not more than 0.006%, Fe and unavoidable impurities, , A hot-formed article provided with a Si-enriched layer satisfying the following formula 4 in the surface layer of the hot-formed article, and a method of manufacturing the same.
[Formula 1] [C] / [Si] 1
(Where each of [C] and [Si] means the content (weight%) of the element)
[Formula 4] [Si] _S1 / [Si] _A1? 3.5
(Si) _S1 means the maximum content (% by weight) of Si in the Si-enriched layer of the hot-formed article, and [Si] _A1 means the average content (% by weight) of Si in the hot-

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a hot-

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-formed article and a method of manufacturing the same, and more particularly, to a hot-formed article which can be suitably applied to an automobile chassis component and a method of manufacturing the same.

In recent years, the use of high-strength steels has been continuously increasing in order to reduce the weight of automobiles. However, if such high-strength steels are processed at room temperature, wear and breakage of the steel sheet are liable to occur and springback phenomenon occurs during machining, There is a problem to lose. Therefore, a so-called Hot Press Forming (HPF) technique has been applied as a preferable method for machining a high strength steel without defects.

The hot press forming (HPF) is a method in which a steel sheet is processed into a complicated shape at a high temperature by utilizing soft properties and high ductility at high temperatures. More specifically, the steel sheet is subjected to a state of austenite region or more And then quenching the steel sheet at the same time as the machining, thereby transforming the structure of the steel sheet into martensite, thereby producing a product having a high-strength and precise shape.

However, if such a high-strength steel is heated to a high temperature, there is a problem that a blister-type scale defect occurs on the surface of the steel as shown in Fig.

Disclosure of Invention Technical Problem [8] The present invention has been made to solve the above-mentioned problems of the prior art, and one of the objects of the present invention is to provide a hot-formed article in which a blistering scale defect is suppressed and a method of manufacturing the same.

One aspect of the present invention is a hot-formed article obtained by hot press forming, wherein the hot-formed article contains 0.25% or less of C (excluding 0%), 0.1 to 0.5% of Si, 0.9 to 1.8% 0.05% or less (excluding 0%), P: 0.05% or less, S: 0.004% or less, N: 0.006% or less, the balance Fe and unavoidable impurities , And a hot-rolled product having a Si-enriched layer satisfying the following formula (4) is provided on the surface layer of the hot-formed article.

[Formula 4] [Si] _S1 / [Si] _A1 ? 3.5

(Si) _S1 means the maximum content (% by weight) of Si in the Si-enriched layer of the hot-formed article, and [Si] _A1 means the average content (% by weight) of Si in the hot-

Another aspect of the present invention is to provide a method of manufacturing a semiconductor device, comprising, by weight%, 0.25% or less of C (excluding 0%), 0.1 to 0.5% of Si, 0.9 to 1.8% of Mn, : Not more than 0.05% (excluding 0%), P: not more than 0.05%, S: not more than 0.004%, N: not more than 0.006%, the balance Fe and unavoidable impurities, sec at a rate of 1 to 10 ° C / sec, secondary heating the primary heated steel to 800 to 980 ° C, heating the secondary heated steel to 800 To 980 占 폚 for 3 to 15 minutes, and cooling the maintained steel material at a rate of 20 占 폚 / sec or more while press-molding the maintained steel material by a die.

As one of the various effects of the present invention, the hot-formed article of the present invention has an advantage that occurrence of a blistering scale defect is effectively suppressed.

It is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

FIG. 1 is a photograph of a surface of a hot-formed article in which a blistering scale defect occurs. FIG.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a hot-formed article as one aspect of the present invention will be described in detail.

First, the alloy components and preferable content ranges of the hot-formed articles will be described in detail. It is to be noted that the content of each component described below is based on weight unless otherwise specified.

C: 0.25% or less (excluding 0%)

C is an element that forms a carbide or solidifies in ferrite to increase its strength. On the other hand, when a scale is formed on the surface layer of a steel sheet, the scale reacts with oxygen (O) in the scale to release carbon monoxide or carbon dioxide gas, so that the scale is peeled from the base steel. If the content exceeds 0.25%, it may react with oxygen (O) in the heating furnace or air cooling to cause excessive scale peeling.

Si: 0.1 to 0.5%

Si is an element to be added to improve strength or ductility, and is added to the extent that there is no surface scale problem during high temperature heat treatment. When the content is less than 0.1%, the adhesion of the iron-based surface scale is small and the peeling is easily caused by forming a relatively large scale. On the other hand, when it exceeds 0.5%, the adhesion of the non-ferrous iron surface scale is increased and the scale removal can be made difficult.

Mn: 0.9 to 1.8%

Mn inhibits ferrite formation and increases the austenite stability to facilitate the formation of low-temperature transformation phases, thereby increasing the strength of the steel. From the viewpoint of scale, when the content is less than 0.9%, the adhesion of the iron-based surface scale is low and peeling can be facilitated. On the other hand, if the content exceeds 1.8%, excessive scale formation may be caused. In addition, it is possible to form a segregation zone inside and / or outside the slab and hot-rolled steel sheet, causing cracks and propagation during performance and / or rolling operations, thereby deteriorating the quality of the steel sheet.

sol.Al: 0.05% or less (excluding 0%)

sol.Al can be concentrated on the surface of the steel sheet to increase the scale adhesion, while inhibiting carbide formation, thereby increasing the ductility of the steel. On the other hand, Al in the steel reacts with N to precipitate AlN to cause slab cracking, which may deteriorate the quality of the cast steel or hot-rolled steel sheet. Therefore, in the present invention, the upper limit of the content of sol.Al is limited to 0.05%.

Ti: 0.05% or less (excluding 0%)

Ti is an element for forming precipitates (TiC, TiCN, TiNbCN) and is an element for increasing the strength of steel. However, if the content exceeds 0.05%, the cost of producing hot-rolled steel sheet may increase, or the yield or the tensile strength may be increased sharply. Which may lead to defective machining.

P: not more than 0.05%

P is an impurity inevitably contained in the steel, and may be segregated in grain boundaries and / or intergranular boundaries to cause brittleness. Furthermore, if the content thereof is excessive, it is considered that it causes surface layer scale defects. Therefore, the content of P should be kept as low as possible, but the upper limit is 0.05%.

S: not more than 0.004%

S is an impurity inevitably contained in steel, which can cause segregation during MnS nonmetallic inclusions and performance solidification in steel and lead to high-temperature cracks. Therefore, the content of S should be kept as low as possible, with the upper limit of 0.004%.

N: not more than 0.006%

N is an austenite stabilization and nitride forming element. However, the nitrogen element dissolved in the steel may react with the alloy element forming the precipitate to reduce the content of the precipitation element required for precipitation strengthening, resulting in a decrease in strength. Therefore, in the present invention, the nitrogen content is kept as low as possible but its content is limited to 0.006% or less.

The rest of the composition is Fe. However, it is not possible to exclude inevitable impurities that are not intended from the raw material or the surrounding environment in a conventional manufacturing process, since they may be inevitably incorporated. These impurities are not specifically referred to in this specification, as they are known to one of ordinary skill in the art.

On the other hand, addition of an effective component other than the above-mentioned composition is not excluded, and may further include, for example, Cr, Mo, Cu, Ni and B.

Cr: 0.3% or less (excluding 0%)

Cr is an element that increases the hardenability of a hot-formed steel sheet. In addition, it plays a role of suppressing the formation of the surface layer scale of the steel sheet when heated at high temperature. However, if the content exceeds 0.3%, excessive curing ability may be caused or the manufacturing cost may increase.

Mo: 0.2% or less (excluding 0%)

Mo is an element that increases the hardenability of the hot-rolled steel sheet and enlarges the austenite temperature section. In addition, it plays a role of suppressing the formation of the surface layer scale of the steel sheet when heated at high temperature. However, if the content exceeds 0.2%, excessive curing ability may be caused or the manufacturing cost may increase.

Cu and Ni: 0.1% or less in total (excluding 0%)

Cu and Ni are elements exhibiting corrosion resistance in a salt spray atmosphere (NaCl liquid or vapor). However, if the content exceeds 0.1%, the degree of reaction with oxygen is low, so that it is concentrated on the surface of the hot-rolled steel sheet to increase the hot-rolled scale adhesion and cause surface defects.

B: 0.005% or less (excluding 0%)

B is an element that increases the hardenability of steel. When added in an appropriate amount, ferrite formation is inhibited and is effective for increasing the hardenability. However, when the content is excessive, the austenite recrystallization temperature is increased and the weldability is deteriorated. On the other hand, in the present invention, since the steel sheet is produced from the electric furnace molten steel, the solid nitrogen content in the steel is high, so that the precipitation strengthening effect by the precipitate forming element can be reduced. Boron (B) is added to form nitrides by reacting with nitrogen dissolved in the steel to increase the precipitation strengthening effect of the steel sheet and to maintain the hardenability of the steel, thereby ensuring proper strength and workability.

On the other hand, it is preferable to control the content of C and Si so as to satisfy the following relational expression 1 when designing an alloy of such a steel material. If the content of C and Si does not satisfy the relational expression (1), surface hardening of the hot-rolled product of Si is insufficient, and the proportion of Fe ₂ O _{3 in the} scale becomes relatively large, and the occurrence of blistering scale defects may increase sharply.

[Relation 1] [C] / [Si] 1

(Where each of [C] and [Si] means the content (weight%) of the element)

In designing the alloy of the steel as described above, it is preferable to control the content of Si and Mn so as to satisfy the following relational expression (2). If the contents of Si and Mn do not satisfy the relationship (2), the blending ratio of the blistering scale may increase rapidly even though the surface concentration of Si is excessively high because the Mn content is excessively high relative to the Si content. In addition, during the electric arc welding, the discharge of the oxide of the welded part is unreasonable, which may lead to weld defect such as pore.

[Relation 2] [Mn] / [Si] 2

(Where each of [Mn] and [Si] means the content (weight%) of the corresponding element)

It is preferable to control the content of C, sol.Al, Cr, and Mo so as to satisfy the following relational expression 3 when designing the alloy of such a steel material. If the contents of C, sol.Al, Cr, and Mo do not satisfy the relationship (3), the amount of the alloy element necessary to resist generation of carbon (C) + oxygen (O) Scale adhesion may be insufficient due to lack of concentration, which may cause blister ring scale defects.

[Relation 3] [C] / ([sol.Al] + [Cr] + [Mo])? 1

(Here, [C], [sol.Al], [Cr] and [Mo] each represent the content (weight%) of the corresponding element)

Hereinafter, other features of the hot-formed article as one aspect of the present invention will be described in detail.

The present inventors have found that to suppress blistering scale defects, the formation of scale should be suppressed during the hot forming process, and it is effective to concentrate Si, which is an element that is more easily oxidized than Fe, on the surface thereof to inhibit scale formation. The surface layer of the hot-formed article according to one aspect of the present invention is provided with a Si-enriched layer satisfying the following formula (4), thereby effectively preventing occurrence of blistering scale defects.

[Formula 4] [Si] _S1 / [Si] _A1? 3.5

(Si) _S1 means the maximum content (% by weight) of Si in the Si-enriched layer of the hot-formed article, and [Si] _A1 means the average content (% by weight) of Si in the hot-

Meanwhile, the Si-enriched layer can be formed not only in the hot forming process of the forming member but also in the manufacturing process of the forming member, and the forming in the hot forming process of the forming member can be more effective in blocking the occurrence of the blistering scale defect. Si is a group 4 element such as C and behaves in a similar manner to C, thereby enhancing the activity of C, stabilizing the ferrite region and reducing the solubility of C, This is because when the Si-enriched layer is formed, the decarburization phenomenon in which the C positioned on the surface of the molded member in the hot forming process is escaped from the surface layer by the oxidation reaction can be accelerated. As will be described later, when this decarburization phenomenon is excessive, the occurrence of blistering scale defects can be accelerated. From the viewpoint of preventing this, it is preferable, but not necessarily, that the maximum content of Si on the surface of the molded member is preferably less than two times, compared to the average content of Si in the molded member before hot forming.

According to further studies by the present inventors, the degree of surface decarburization of the hot-rolled products also influences the occurrence of blistering scale defects. The decarburization on the surface of the hot-rolled product means that the C located on the surface of the hot-rolled product has escaped to the outside by the oxidation reaction. The oxidized C thus generates a gas such as CO or CO ₂ , Resulting in the occurrence of blistering defects. According to one example, the maximum surface decarburization depth of the hot-formed article may be 30 mu m or less, and in this case, the occurrence of the blistering defect can be more effectively suppressed.

According to further research results of the present inventors, the weight ratio of Fe ₂ O ₃ and Fe ₃ O ₄ contained in the scale layer formed on the surface of the hot-rolled product also affects occurrence of blistering scale defects. It is considered that the scale layer is relatively dense when the Fe ₂ O ₃ fraction is high in the scale constitution phase and it is presumed that the gas generated at the Fe / scale interface is not efficiently discharged from the surface of the molded article. According to one example, the weight ratio of Fe ₂ O ₃ and Fe ₃ O ₄ contained in the scale layer may be 1: 9 to 3: 7, in which case the occurrence of blistering defects can be more effectively suppressed.

According to further research results of the present inventors, the thickness of the scale layer formed on the hot-rolled product surface also affects the occurrence of blistering scale defects. Particularly, the thickness of the scale layer has different effects when the hot-rolled steel sheet is hot-pressed and when the cold-rolled steel sheet is hot-pressed. If the hot-rolled steel sheet is hot-pressed and the thickness of the scale layer is less than 13 占 퐉 The occurrence of blistering defects is more effectively suppressed. When the cold-rolled steel sheet is hot-pressed, the occurrence of blistering defects is more effectively suppressed when the thickness of the scale layer is 5 占 퐉 or more.

Hereinafter, a method for manufacturing a hot-formed article, which is another aspect of the present invention, will be described in detail.

First, after the forming member having the above-mentioned component system is prepared, the forming member is firstly heated to 500 to 750 ° C at a rate of 0.1 to 20 ° C / sec.

This step is a step for suppressing the diffusion of carbon (C) atoms before the scale of a certain thickness is formed on the surface of the molded member, other than the purpose of quickly reaching the temperature of the molded member to the target temperature. The thickness of the scale formed on the surface of the molded part is oxygen (O) there is associated with the concentration directly, in the present invention, scale adhesion increase or scale blistering occurs is to be minimized oxygen (O) concentration was at a low N ₂ atmosphere at In order to form a thin scale thickness on the surface of the molded member by primary heating, the heating rate in the present invention is limited to 0.1 ° C / sec or more. On the other hand, when the heating rate is lowered at a rate of less than 0.1 ° C / sec, the time for producing the molded part becomes very long, and the decaburization reaction due to the surface diffusion of carbon (C) atoms and the reaction with oxygen (O) There is a difficulty in minimizing. A more preferable lower limit is 5 占 폚 / sec, and a still more preferable lower limit is 10 占 폚 / sec. On the other hand, if the heating temperature is higher than 20 ° C / sec, it is impossible to implement a heating furnace of induction heating system which heats a conventional molded member, and a separate heating apparatus is required to be manufactured.

Next, the primary heated molded member is heated to 800 to 980 ° C at a rate of 1 to 10 ° C / sec, followed by holding for 3 to 15 minutes.

In this step, in addition to the purpose of slowing the heating rate and preventing the temperature of the molded member from reaching the target temperature or more, the increase of the surface concentration of the silicon (Si) element on the interface between the scale formed on the surface of the molded member and the base steel is increased It is a step to increase the scale adhesion. This is related to the time for heating and holding the molded member at the target temperature. The diffusion rate of the silicon (Si) element is slower than the carbon (C) element at the same temperature and time condition as the carbon (O) existing in the furnace atmosphere, it is considered that the content of the silicon (Si) element concentrated on the surface of the molded member can be increased. Therefore, the surface decarburization of the carbon (C) element can be minimized while the silicon (Si) element is concentrated on the surface of the hot-formed member. On the other hand, a more preferable upper limit of the secondary heating rate is 8 占 폚 / sec, and a still more preferable upper limit is 5 占 폚 / sec.

Next, if necessary, the retained molded member is cooled in the air to a temperature equal to or higher than Ar3 at a rate of 10 DEG C / sec or less. The reason for cooling in the atmosphere is that the physical time for moving the molded member before hot forming the molded member and the time for the upper punching to move for mold cooling are required.

Next, the held formed member is press-molded by a die and cooled at a rate of 20 DEG C / sec or more to produce a hot-formed article.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the description of these embodiments is intended only to illustrate the practice of the present invention, but the present invention is not limited thereto. And the scope of the present invention is determined by the matters described in the claims and the matters reasonably deduced therefrom.

( Example )

A cold-rolled steel sheet having the composition shown in Tables 1 and 2 below was prepared and heated to 700 占 폚 at a rate of 12 占 폚 / sec in a nitrogen (N2) atmosphere, and then heated to 900 占 폚 at a rate of 3 占 sec And maintained for 5 minutes. Thereafter, the steel sheet was cooled to a temperature of Ar 3 + 10 ° C in an atmospheric air at a rate of 5 ° C / sec, followed by press molding by a mold and quenching.

Then, the thickness and composition ratio of the scale layer were measured for each of the manufactured hot-rolled products, and the maximum content of Si and the maximum surface decarburization depth in the Si-enriched layer were measured and visually evaluated for occurrence of blistering defects. The results are shown in Table 3 below.

Steel grade Alloy component (% by weight) C Si Mn P S Sol.Al Cr Mo A 0.226 0.252 1.303 0.0131 0.0032 0.042 0.1475 - B 0.22 0.377 1.295 0.0127 0.0029 0.0361 0.1491 - C 0.226 0.25 0.910 0.0126 0.0022 0.0408 0.1487 - D 0.231 0.257 1.619 0.0145 0.0035 0.0438 0.1516 - E 0.241 0.496 1.298 0.0135 0.0027 0.012 0.1122 0.115 F 0.238 0.196 1.236 0.0128 0.0024 0.0342 0.1435 0.1493 G 0.245 0.202 0.811 0.0125 0.002 0.0326 0.1472 0.1545 H 0.247 0.205 1.604 0.0134 0.0028 0.0334 0.1483 0.1582

Steel grade Alloy component (% by weight) Relationship 1 Relation 2 Relation 3 Ti Cu Ni B N A 0.0302 - - - 0.0035 0.90 5.17 1.19 B 0.0294 - - - 0.0026 0.44 2.62 1.19 C 0.0302 - - - 0.0033 0.90 3.64 1.19 D 0.0311 - - - 0.0032 0.90 6.30 1.18 E 0.0397 0.0012 0.0032 0.0025 0.0039 0.49 2.62 1.01 F 0.0393 0.0012 0.0032 0.0023 0.0043 1.21 6.31 0.73 G 0.0407 0.0012 0.0032 0.0024 0.003 1.21 4.01 0.73 H 0.0405 0.0012 0.0032 0.0024 0.0048 1.20 7.82 0.73

Steel grade Scale layer Si thick layer Maximum surface decarburization depth Blistering defect Remarks Composition ratio thickness Si maximum content Equation 4 Value A 1.5: 8.5 6.0 1.39 5.52 13 X Inventory 1 B 1.6: 8.4 6.3 1.34 3.55 17 X Inventory 2 C 1.9: 8.1 7.2 1.68 6.72 11 X Inventory 3 D 1.4: 8.6 7.7 1.46 5.68 15 X Honorable 4 E 1.8: 8.2 6.8 1.74 3.51 21 X Inventory 5 F 3.6: 6.4 4.2 0.61 3.11 16 O Comparative Example 1 G 3.7: 6.3 8.2 0.63 3.12 17 O Comparative Example 2 H 1.6: 8.4 3.8 0.61 2.98 23 O Comparative Example 3

In the case of Inventive Examples 1 to 5, which satisfy all of the conditions proposed in the present invention, the maximum Si content in the Si-enriched layer satisfies the range suggested by the present invention, and the composition ratio of the scale layer and the like It can be confirmed that it has been properly managed, and as a result, it can be confirmed that the blistering defect does not occur.

On the other hand, in Comparative Examples 1 to 3, the maximum Si content in the Si-enriched layer exceeded the range proposed by the present invention, and thus a blistering defect occurred.

Claims (12)

A hot-rolled product obtained by hot press forming, wherein the hot-rolled product contains 0.25% or less of C (excluding 0%), 0.1 to 0.5% of Si, 0.9 to 1.8% of Mn, (Excluding 0%), Ti: not more than 0.05% (excluding 0%), P: not more than 0.05%, S: not more than 0.004%, N: not more than 0.006%, Fe and unavoidable impurities, , And a Si-enriched layer satisfying the following formula (4) is provided on the surface layer of the hot-formed article.
[Formula 1] [C] / [Si] 1
(Where each of [C] and [Si] means the content (weight%) of the element)
[Formula 4] [Si] _S1 / [Si] _A1? 3.5
(Si) _S1 means the maximum content (% by weight) of Si in the Si-enriched layer of the hot-formed article, and [Si] _A1 means the average content (% by weight) of Si in the hot-
The method according to claim 1,
At least one selected from the group consisting of Cr: not more than 0.3% (excluding 0%), Mo: not more than 0.2% (excluding 0%), Cu and Ni: not more than 0.1% ≪ / RTI >
3. The method of claim 2,
By weight, and B: 0.005% or less (excluding 0%).
delete The method according to claim 1,
The hot-molded article satisfies the following formula (2).
[Formula 2] [Mn] / [Si]? 2
(Where each of [Mn] and [Si] means the content (weight%) of the corresponding element)
3. The method of claim 2,
The hot-molded article satisfies the following formula (3).
[Formula 3] [C] / ([sol.Al] + [Cr] + [Mo])? 1
(Here, [C], [sol.Al], [Cr] and [Mo] each represent the content (weight%) of the corresponding element)
The method according to claim 1,
Wherein the maximum depth of surface decarburization of the hot-formed article is 30 占 퐉 or less.
The method according to claim 1,
The hot-rolled product further comprises a scale layer containing Fe ₂ O ₃ and Fe ₃ O ₄ on its surface, wherein the weight ratio of Fe ₂ O ₃ and Fe ₃ O ₄ contained in the scale layer is 1: 9 to 3: 7 hot-rolled products.
The method according to claim 1,
The hot-formed article is obtained by hot-pressing a hot-rolled steel sheet, and the thickness of the scale layer formed on the surface of the hot-formed article is less than 13 占 퐉.
The method according to claim 1,
Wherein the hot-rolled product is obtained by hot-pressing a cold-rolled steel sheet, and the thickness of the scale layer formed on the surface of the hot-rolled product is 5 占 퐉 or more.
Al: 0.05% or less (excluding 0%), Ti: 0.05% or less (0% or less), C: not more than 0.25% ), P: not more than 0.05%, S: not more than 0.004%, N: not more than 0.006%, the balance Fe and unavoidable impurities;
Heating the molded member to 500 to 750 ° C at a rate of 0.1 to 20 ° C / sec;
Secondarily heating the molded primary member to 800 to 980 占 폚 at a rate of 1 to 10 占 폚 / sec;
Maintaining the secondary heated shaped member at 800 to 980 캜 for 3 to 15 minutes; And
Molding the held formed member by a die and cooling at a rate of 20 ° C / sec or more;
Of the hot-rolled product.
[Formula 1] [C] / [Si] 1
(Where each of [C] and [Si] means the content (weight%) of the element)
12. The method of claim 11,
Further comprising the step of cooling the retained molded member to a temperature equal to or higher than Ar3 at a rate of 10 DEG C / sec or less before press molding.

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